The present invention relates to medical diagnostic devices, more particularly to devices that measure respiratory parameters.
It is often desirable to measure respiratory parameters to monitor and diagnose the progression of respiratory diseases and to make beneficial therapeutic recommendations. It is also desirable to measure respiratory parameters in individuals who have been exposed to smoke, biological or chemical substances, and to determine and prescribe appropriate treatment.
Respiratory parameters are based on a subject's ability to inhale or exhale. A subject's ability to produce airflow or pressure differentials, and thereby create measurable respiratory parameters, is significantly influenced and controlled by the subject's entire respiratory system, that is, the subject's chest wall musculature, diaphragm, lung parenchymal tissue and airway structure. Accordingly, it is difficult to measure respiratory parameters as they relate to specific organs or parts of organs, for example, the lungs or the airways. As a result, most attempted measurements of lung- or airway-specific respiratory parameters, for example, airway resistance (Raw), airway conductance (Caw), lung compliance, and total lung capacity (TLC) are at times inaccurate and do not reflect the actual lung and/or airway performance of the subject. Sometimes this inaccurate measurement may lead to misdiagnosis, or the prescription of unnecessary or harmful drugs or therapy.
Several devices are presently used to measure respiratory parameters of subjects, however, each device is based on airflow and pressure differentials generated by the entire respiratory system, and therefore are not lung- or airway-specific. One such device is a spirometer. A spirometer measures airflow rates and air volumes with a sensor as subject inhales or exhales through a mouthpiece on the spirometer. Another respiratory measurement device is the plethysmograph, which specifically measures respiratory parameters of airway resistance and airway conductance. The plethysmograph includes a large chamber in which a subject is enclosed. In operation, the subject pants within the chamber to create pressure changes in the chamber. The pressure changes are correlated to airflow changes within the subject's respiratory system to calculate Raw and Caw using an application of Boyle's law (P1V1=P2V2). In addition to being space prohibitive due to its large, typically immobile chamber, like the spirometer, the plethysmograph is not lung- or airway-specific because the pressure and airflow changes inside the chamber are created by the subject's entire respiratory system.
A third device that measures respiratory parameters is included in a hospital-grade ventilator. U.S. Pat. No. 6,257,234 to Sun discloses this device, which superimposes a momentary sinusoidal pressure oscillation on the air pressure provided by the ventilator to the subject as the ventilator supports inspiration by the subject. The momentary sinusoidal pressure oscillation is evaluated to determine and calculate airway resistance. Although Sun may be used to measure airway resistance, it suffers several shortcomings. First, it is only effective for evaluating respiratory resistance in a ventilated subject. Second, if the forced impulse oscillation is introduced at the wrong time during inspiration, the collected data may be inaccurate. Further, the forced impulse oscillation provides only an instantaneous measurement of the subject's respiratory system, which may be inaccurate due to chest wall musculature fatigue or contraction, or the state of the lung parenchymal tissue at that specific moment the oscillation is imposed.